Added extra CMAKE flags for linking plugins statically

[agruzdev]

Currently for statically linked backends there are used only two variables: LIBDE265_INCLUDE_DIR and LIBDE265_LIBRARY, for example. This is not enough for multiconfig build systems, like MSVC, where we need to provide both 'debug' and 'release' libraries.

This commit adds support of optional cmake variables:

• plugin_DEBUG_LIBRARY - if not empty, then target_link_libraries will be called twice:

target_link_libraries(heif PRIVATE debug ${${varName}_DEBUG_LIBRARY})
target_link_libraries(heif PRIVATE optimized ${${varName}_LIBRARIES})

• plugin_LINK_DIRS - willl be passed to target_link_directories( )

This change is backward compatible. In case if the new variables empty, behaviour is not changed.